1. Field of the Invention
This invention relates to scanning tunneling microscopes and, more particularly, to such microscopes which correct for the effects of scan drive and/or bias voltages on the tunneling current.
2. Description of the Prior Art
Scanning tunneling microscopes are devices which provide three-dimensional topographic images of surfaces. These devices are capable of providing resolution to atomic dimensions of surface features. In a typical scanning tunneling microscope, an extremely sharp, conducting tip is positioned two to three atomic diameters (typically ten angstroms) above a sample. If the sample surface is biased at a small voltage relative to the tip, then a current caused by the tunneling effect will flow between the sample and the tip when the tip is within atomic distance of the sample. This current is a function of the distance between the tip and the sample. A microscope of this type is described in U.S. Pat. No. 4,343,993 of Binnig et al. The sample is either a conductor, a semiconductor, or an insulator that has been thinly coated with a conductive material. Scanning tunneling microscopes require a conducting surface for operation. In some special cases, it is possible to image a non-conducting material, if it is very thin (a few atoms thick) and on a conducting surface. For instance, DNA molecules on graphite have been imaged.
The tip in a scanning tunneling microscope must be positioned with extreme accuracy in three dimensions relative to a sample. Motion perpendicular to the sample (z-axis) provides surface profile data. Motion parallel to the surface generates the scanning. In a typical system, the image is developed from a raster type scan, with a series of data points collected by scanning the tip along a line (x-axis), and displacing the tip perpendicularly in the image plane (y-axis), and repeating the step and scan process until the image is complete. The precise positioning along the x, y and z axes required to generate atomic scale images is usually accomplished with a piezoelectric transducer device. Piezoelectric devices can be made to expand or contract by applying voltages to electrodes that are placed on the piezoelectric material. The motions produced can be extremely small, with sensitivities as low as tens of angstroms per volt. The maximum deflection possible for these types of scanners is currently about 100 microns. Scanners with different deflection coefficients are used for different applications, with small coefficients used for atomic resolution images, and larger coefficient scanners used for lower resolution, larger area images. The design of the piezoelectric scanner, including the shape of the scanner and the placement of electrodes is well known in the art. Typically piezoelectric scanners for scanning tunneling microscopes have voltages as high as several hundred volts applied to them.
In a scanning tunneling microscope, either the sample can be attached to the scanner, and the tip held stationary, or the tip can be attached to the scanner, and the sample fixed. As the tip is scanned in the x and y directions, the z axis movement is closely coupled to the parameter sensed by the tip. The image is generated in two ways. In one method, the z position can be varied as the tip is scanned to maintain constant tunneling current, and the surface profile can be derived from the z scan voltage. In this method, for each x, y position, the z drive voltage corresponds to the height of the tip required to maintain constant tunneling current. The other method works by maintaining the tip at a constant z position and monitoring the variation in tunneling current as the tip is scanned in the x and y directions.
A scanning tunneling microscope can also be used for other functions besides surface profile measurement. For example, one can hold the tip stationary and vary the bias voltage applied to the sample, while monitoring the tunneling current. In this fashion local I-V characteristics can be obtained, with the same spatial resolution as can be achieved for profile data.
A typical piezoelectric scanner for a scanning tunneling microscope is a hollow tube made of piezoelectric material with a diameter of about one-half inch. The tube will have electrodes attached to various parts of its surface, which cause the tube to contract or expand and generate scanning motions when voltages are applied to the electrodes. The placement of the electrodes is dependent on the type of scanning application. The tip can be placed either in the center of the scan tube or on the rim, depending on the application. Because of the high voltages applied to the electrodes, any coupling between the electrodes and the tunneling signal can have undesirable effects. For one such scanner, the capacitive coupling between the tip and its associated connections to the z electrode, with no shielding, was observed to be on the order of half a picofarad, a number which was both calculated and measured. Shielding the electrode and the tip wire can reduce this number by about a factor of ten to 20-50 millipicofarads. Similar coupling exists between the tunneling current signal path and the x and y drive electrodes.
In addition, there may be capacitive coupling between the tunneling current signal and the sample and sample holder. This coupling will cause an effect on the tunneling current signal when the bias voltage is changed. This effect is most noticeable when I-V data is being taken.
The scanner drive voltage for z typically can slew at several volts per microsecond. Given a one volt per microsecond slew and a 40 millipicofarad coupling, and assuming no other parasitic effects, one can use the following relationship to calculate the induced tunneling current error: EQU I=C dV/dT
An induced current error of four nanoamps would be observed on the tunneling current signal, a number which is quite large compared to typical tunneling current values. The coupling to the z electrode is the most serious effect, as tunneling current and z drive voltage are connected by the feedback loop, and the induced error can lead to loop instabilities or data inaccuracies. Coupling to the x electrode is less serious, as the x drive voltage is usually a linear ramp, so the induced effect is a DC offset. However for bidirectional scanning, the x coupling causes a different offset for forward and reverse scans, an effect which is easily observed in most scanning tunneling microscopes.
The above example assumes a coupling capacitance due to the geometric configuration of the scanner that is independent of frequency. This analysis accurately describes the coupling between scan drive voltage and tunneling current signal when the primary coupling factor is the geometric capacitance inherent in the configuration of the scanner. For some scanner configurations, or as the systems become faster, with faster sampling and slew rates, smaller parasitic R and C elements can contribute significantly to the coupling effect.
As scan sizes become larger, the amplitude and slew rate of the z drive voltage increase, because to achieve large scans at high scan rates, the z drive must have the capability to follow the surface topography at high speed. As the amplitude and slew rate increase, the amount of parasitic and geometric coupling that can be tolerated between the tunneling current and the scanner drive voltages becomes extremely small, too small to eliminate entirely by the physical and electrical design of the scanner. Therefore, the ability to correct for the coupling effect would be extremely useful for improving the performance of scanning tunneling microscopes.